1. Field of the Invention
The present invention relates to a connecting device for high-voltage cables used in internal combustion engines of, for example, automobiles, etc. More particularly, the invention is used to connect a spark plug to a high-voltage cable extending from an ignition coil or a distributor in an internal combustion engine.
2. Description of Background Information
Such an electrical connecting device, a plug connection, an ignition coil or distributor connection are known, and the plug connection may have, for example, the structure shown in FIG. 1. One such example is described in U.S. Pat. No. 5,348,486, issued on Sep. 20, 1994.
The above plug connection comprises a socket terminal 2 made of stainless steel or other similar materials. At one end, the socket terminal 2 is electrically connected to the high-voltage cable 1. The plug connection further comprises a flexible elastomeric boot 3 made of an insulating and elastic material such as silicone rubber, for housing the socket terminal 2.
The socket terminal 2 comprises, at one end, a cable-connecting portion 2a in which the high-voltage cable 1 is inserted and pressed by the cable connecting portion to ensure electrical connection. At the other end, a plug-connection portion 2b receives the head portion 5a of the spark plug 5, which is inserted and plugged therein from a direction perpendicular to the axis of the cable-connecting portion 2a. The head portion 5a of this spark plug 5 has a shape similar to a bulb.
An intermediate portion of the socket terminal between the cable-connection portion 2a and the plug-connecting portion 2b forms a constricted portion 2c which has a narrower width.
The plug-connecting portion 2b comprises a plug-side socket opening 2d, into or from which the head portion 5a can be plugged or unplugged, and a C-shaped elastic ring 2e cooperating with the plug-side socket opening 2d.
The plug-side socket opening 2d has a pair of holes (not shown in the figures), provided at substantially opposed positions.
The elastic ring 2e includes, at each end thereof, a pair of projections 2f which project inwardly, as shown in FIG. 5.
When the ring 2e is fitted around the plug-side socket opening 2d from outside, each projection 2f is snapped into the corresponding hole of the plug-side socket opening 2d. Thus, the ring 2e is fixed around the plug-side socket end with each projection 2f projecting inwardly.
The elastomeric boot 3 is formed in an L-shape and comprises a terminal container 3a for housing the socket terminal 2, an opening 3b for receiving the high-voltage cable 1 therein, and a plug-receiving portion 3c, through which the head 5a of the plug is guided.
FIG. 1 shows a process in which the elastomeric boot 3 is connected to the spark plug 5 installed in an engine via the head portion 5a. In this construction, the projections 2f of the elastic ring 2e are fitted into the neck 5b adjacent to the head 5a, so that the spark plug is prevented from being released.
However, the plug connections of the prior art have drawbacks when the spark plug 5 is inserted therein, as shown, for example, in FIG. 2. When the spark plug is inserted in an oblique position relative to the axis of the plug-receiving portion 3c, the edge of the head portion 5a may abut against the rim of the plug-side socket opening 2d or be hooked thereby, so that the head portion 5a is not snugly installed therein or cannot be installed therein.
Further, the elastomeric boot 3 is deformable and expandable due to its elasticity. As shown in FIG. 3, the elastic deformation tends to increase the area of the region where the head portion 5a and the rim of the plug-side socket opening 2d come into contact, thereby further worsening the fittability.
FIG. 4 shows the case when the elastic ring 2e is tightly set. When the socket terminal 2 is removed from the spark plug 5 in such a case, the socket terminal 2 may be inadvertently bent inside the elastomeric boot. Then, a great force is required to remove the head portion from the socket terminal. When forcibly removed, this may cause deformations of the socket terminal 2, as shown in FIG. 5.
Likewise, the known elastomeric boot is bent into an L-shape, so that the inside corner 3d, located between the terminal container 3a for housing the socket terminal and the plug-receiving portion 3c for introducing the plug, forms a semi-circle.
In the above construction, when the spark plug is inserted in an oblique direction in the plug-receiving portion 3c as shown in FIG. 6, the head portion 5a of the spark plug 5 may be caught by the constricted portion 2c of the socket terminal 2. Then, the head portion cannot be placed correctly, but lies in the constricted portion, as shown by dotted lines in FIG. 7.
Furthermore, the constricted portion 2c of the socket terminal has a relatively deep cut-away portion extending from the periphery towards the axis of the socket terminal 2, as shown in FIGS. 1 and 6. In addition, the inside corner portion 3d has a round shape. In such a case, when removing the socket terminal 2 from the plug, the socket terminal 2 tends to tilt inside the boot 3, due to a strong fitting force of the elastic ring 2e. This phenomenon may be worsened by an unstable fixing of the socket terminal 2 inside the terminal container 3a, and eventually, by a deformation of the socket terminal 2. Also, a strong releasing force is required for removal of the plug connection from the spark plug (FIG. 4). When the spark plug is forcibly removed, the socket terminal 2 may be deformed as shown in FIG. 5.
Also, in the known plug connection of FIGS. 1 to 6, the opposed rim portions of the plug-side socket opening 2d, which are spaced-apart from one another at the open side of the elastic ring 2e, are also separated from the inside corner 3d of the socket terminal 2. Moreover, the separation between the rim portions is rather large at the open side of the elastic ring and, further, increases from the closed end of the plug-connecting portion towards the opening 2d thereof.
Therefore, as shown in FIG. 6, when the plug is introduced in an oblique direction through the plug passage 3c of the boot 3, the head portion 5a of the spark plug may be inadvertently caught by the constricted portion 2c of the socket terminal 2. The head portion 5a may then not be fitted properly into the plug-connecting portion 2b, as shown by the dotted line in FIG. 7.